Only about half of the heavy metal in the sea results from rainfall. The other half comes from gaseous sources.
Each year coal-fired plants worldwide emit vast quantities of mercury, of which some 2,000 metric tons enters into the world’s oceans in gas form. Once it is in seawater, the heavy metal, which can adopt various chemical forms, can pose hazards to marine ecosystems.
Mercury also poses hazards to people’s health because it enters the food chain by accumulating in the tissue of fish and other animals in the form of methylmercury. This highly toxic substance can cause a variety of health conditions such as impeded brain development in children and cardiovascular diseases in adults.
Alarmingly, industrial activities have tripled the amount of mercury in the surface of the oceans over the past couple of centuries. It turns out, however, that mercury enters the oceans not only via rainfall, as previously thought, but also through gas exchange, according to experts at the University of Basel.
And that’s good news in a way because targeted measures to tackle mercury pollution coud work more effectively once we understand its mechanisms better.
The scientists, who report their findings in the journal Nature, analyzed seawater samples from various depths in the Mediterranean Sea and the North Atlantic.by help of a method called fingerprinting that allowed them to tell whether mercury contained therein originated from rainfall or gas exchange.
They have discovered that only about half of the heavy metal in the sea results from rainfall while the other half enters the oceans in a gaseous form, which led them to conclude that we have generally overestimated the contribution of precipitation to mercury pollution.
This finding is important because “if less mercury enters the sea via rainfall, a reduction in emissions could cause mercury levels in seawater to drop faster than anticipated,” explains Martin Jiskra, a biogeochemist at the Department of Environmental Sciences at the University of Basel.
However, mercury also enters oceans, near coastal areas, via another unsuspected source: rivers. In fact, other researchers have found, rivers are the main sources of mercury pollution in coastal areas from where the heavy metal then moves further out into the oceans, according to experts in the United States.
“It kind of rewires the global mercury cycle,” says Peter Raymond, a professor of ecosystem ecology at the Yale School of the Environment who was an author of a new study published in Nature Geoscience.
This means that the current focus on controlling atmospheric emission and deposition of mercury is less effective in tackling mercury pollution because it is based on a flawed understanding. Rather, controlling emission from industries situated alongside certain rivers is a must.
This is especially so because just 10 rivers are responsible for half of the mercury pollution in oceans carried by rivers. The three main riverine polluters in the world are the Amazon in South America, the Ganges in India and Bangladesh, and the Yangtze in China, the scientists note.